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Date: Fri, 17 Jul 2015 15:12:32 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used
 during runtime
Message-ID: <20150717131232.GK19282@twins.programming.kicks-ass.net>
References: <1437135724-20110-1-git-send-email-mgorman@suse.de>
 <1437135724-20110-4-git-send-email-mgorman@suse.de>
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In-Reply-To: <1437135724-20110-4-git-send-email-mgorman@suse.de>
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>, Nicolai Stange <nicstange@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Alex Ng <alexng@microsoft.com>, Fengguang Wu <fengguang.wu@intel.com>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Fri, Jul 17, 2015 at 01:22:04PM +0100, Mel Gorman wrote:
>  int __meminit early_pfn_to_nid(unsigned long pfn)
>  {
> +	static DEFINE_SPINLOCK(early_pfn_lock);
>  	int nid;
>  
> -	/* The system will behave unpredictably otherwise */
> -	BUG_ON(system_state != SYSTEM_BOOTING);
> +	/* Avoid locking overhead during boot but hotplug must lock */
> +	if (system_state != SYSTEM_BOOTING)
> +		spin_lock(&early_pfn_lock);
>  
>  	nid = __early_pfn_to_nid(pfn, &early_pfnnid_cache);
> -	if (nid >= 0)
> -		return nid;
> -	/* just returns 0 */
> -	return 0;
> +	if (nid < 0)
> +		nid = 0;
> +
> +	if (system_state != SYSTEM_BOOTING)
> +		spin_unlock(&early_pfn_lock);
> +
> +	return nid;
>  }

Why the conditional locking?

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